1. Field of the Invention
The present invention relates generally to a surface processing technique for removing, by etching, metals and inorganic substances from a surface of a substrate or other material to be processed and more particularly to a technique for forming transparent electrodes of an Indium Tin Oxide (ITO) film or wiring on a substrate used for fabrication of semiconductor devices, such as, for example, liquid crystal displays (LCD).
2. Description of the Related Art
Wet etching methods using an organic solvent and dry etching methods are well known as methods for removing oxides from surfaces of a substrate including a wafer, an IC chip and any other of various materials to be processed and typically used for fabrication of semiconductor devices. For example, when a desired electrode or wiring is formed on a glass substrate by etching an ITO (Indium Tin Oxide) film, which is widely used as a transparent conductive film in, for example, LCD, or when excessive ITO film is to be removed from a substrate, the wet etching method is generally used.
In recent years, however, increasingly fine etching has been required for complying with the demands for higher definition of a liquid crystal display device, and thus the dry etching method has increasingly become important. The dry etching method generally includes etching a surface of a material to be processed, such as a substrate, by using a plasma which is generated by gas discharge in a vacuum or at a reduced pressure. Additionally, an etching method has recently been developed in which plasma discharge is generated at a pressure close to or at atmospheric pressure, and therefore without the need for special equipment such as a vacuum chamber and a vacuum pump, thereby simplifying the surface treatment apparatus used and decreasing the size thereof. For example, Japanese Patent Laid-Open No. 2-15171 discloses a method in which a reactive gas mixture containing rare gas and organic monomer gas is introduced into a reactor, and a gas discharge plasma is generated at a pressure close to or at atmospheric pressure which is used to process a surface of a substrate. This reference also describes the plasma-etched surface obtained by the monomer gas and the other reaction conditions used.
The applicant of this application has also proposed a method disclosed in International Patent Application, Publication No. WO94/22628 in which an oxide (SiO.sub.2) on a substrate surface is etched with active gas species, which are generated by gas discharge at a pressure close to or at atmospheric pressure. In this reference, a mixture containing a rare gas, compressed air or oxygen and a fluorine compound (CF.sub.4, C.sub.2 F.sub.6, CF.sub.6, or the like) is used as a reactive gas mixture. Thus, in this method, for example, when a reactive gas mixture containing tetrafluorocarbon (CF.sub.4) is used as a gas capable of discharge for removing an oxide film from a substrate, etching is carried out, as shown by the reaction Formula I below. EQU SiO.sub.2 +CF.sub.4 .fwdarw.SiF.sub.4 (g)+CO.sub.2 (g) (I)
Thus, SiO.sub.2 is fluorinated to SiF.sub.4 which is easily removed by vaporization because of its relatively low boiling point (about 200.degree. C.).
Conventional wet methods for etching an ITO film, as shown in the prior art, although satisfactory, suffer from the disadvantage that it is difficult to control a particular worked shape, and it is difficult to produce a uniform planer distribution of the etching solution, particularly, when the substrate has a large area to be processed. The wet method thus suffers from the disadvantage of causing nonuniformity in the degree of etching. The wet method also suffers from the disadvantage of treating and disposing the waste solution after etching, and thus the removed oxide in the waste solution may again adhere to the substrate. Particularly, in a TFT (Thin Film Transistor) type LCD, the wet method causes the problem that an Al (Aluminum) electrode and wiring provided below the ITO film to be removed may be corroded with the etching solution.
The conventional dry etching methods in a vacuum or under reduced pressure not only have a relatively high production cost and low throughput, but may also produce a toxic or harmful gas of an In (Indium) compound by chemical reaction, and additionally, has the possibility of thermal explosion due to the use of methanol. The dry etching methods are therefore also unsatisfactory.
On the other hand, the above-described plasma etching methods at a pressure close to or at atmospheric pressure suffer from the disadvantage that it is difficult to remove metals or metallic compounds containing metals such as gold (Au), indium (In), tin (Sn) and the like, whose fluoride products of the chemical reaction with the reactive gas mixture hardly vaporize. The plasma etching method thus cannot be employed efficiently for etching an ITO film which may be used as a transparent electrode film of a substrate in fabrication of LCD.